1. Field of the Invention
The invention relates to the cryogenic separation of air. More particularly, it relates to the pretreatment of feed air to cryogenic air separation systems.
2. Description of the Prior Art
Nitrogen and oxygen are desired for many chemical processing, refinery, metal production and other industrial applications. While various techniques are known for the production of nitrogen and/or oxygen by air separation, cryogenic distillation processes and systems are widely used for the production of nitrogen and/or oxygen from air, or for the removal of nitrogen from well gases. In each cryogenic application, high freezing point contaminants, which would otherwise solidify at the low temperatures at which the primary gas separation takes place, must be removed from the compressed feed gas stream. Such contaminants are commonly removed by refrigeration/adsorption process combinations well known in the art. In air separation operations, this pre cleanup can utilize a reversing heat exchanger and cold end gel trap combination, or a mechanical air chiller/zeolite molecular sieve adsorber combination. In the former type of processing unit, virtually all of the contaminants are frozen out of the feed air when the feed air when said air is thermally exchanged against the cryogenic waste and product gas streams. Unfortunately, however, the self cleaning of the reversing heat exchanger unit requires a large purge gas flow relative to the air feed. As a result, the air recovery of such cleanup cycles tends to be undesirably limited Reversing heat exchanger units also require large valves, which must open and close on a cyclic basis, switching the air feed and waste purge flow passages. The valves are often located within the insulated cold box portion of the cryogenic system, making maintenance difficult. Furthermore, to act effectively, the heat exchange-gel trap combination must operate at low temperature, and thus requires a considerable cool down period during plant start-up.
In contrast to reversing heat exchanger and gel trap combinations, mechanical chiller/adsorptive unit combinations, as disclosed in Prentice, U.S. 4,375,367, can supply a clean, dry feed air stream within minutes of start-up. The mechanical chiller reduces the air temperature to about 40.degree. F. from the compressor aftercooler temperature of from about 80.degree. F. to about 115.degree. F. The air, which is saturated at the higher temperatures, loses the bulk of its water burden through condensation, thus reducing the inlet water concentration to the adsorptive unit. The adsorption operation is typically carried out using a pair of pressure vessels, one bed being used for adsorbing purposes, while the other is undergoing regeneration. The pressure vessels are filled with an adsorbent material, such as alumina, zeolite molecular sieve or silica gel, which removes the remaining water vapor, carbon dioxide and/or other contaminants from the feed air stream. The adsorbent beds are usually regenerated at near ambient pressure with a contaminant free stream, either a portion of the cryogenic waste or dry air, which may be heated to improve its desorbing capability. The operation of the mechanical chiller substantially improves the performance of the adsorber beds by increasing their adsorption capacity, reducing the inlet water concentration, and, consequently, the purge flow and energy requirements of the operation. The mechanical chiller is limited to a minimum product dewpoint of about 38.degree. F. due to the necessity for avoiding the buildup of ice on the tubing walls. The chillers must also be followed by a moisture separator to remove the condensate formed from the feed air and to protect the adsorbent beds from excessive moisture. The mechanical chillers used in such operations tend to be expensive in terms of capital and power requirements, especially for small plants. In addition, such chillers are generally known for requiring expensive maintenance.
In light of such factors, there has been a desire in the art for new processes and systems that would either eliminate or modify the function of the components referred to above, particularly the mechanical chiller and moisture separator so as to more economically provide clean, dry air to a cryogenic gas separation unit. One approach considered with interest is the use of membrane systems to selectively permeate water from feed air. Certain materials are well known as being capable of selectively permeating water, while air or other gases, comprising less permeable components, are recovered as non-permeate gas. A membrane system utilizing such a material would replace the function of the mechanical chiller. Such membrane systems are well known to be relatively simple and easy to operate and maintain. As such membrane systems are normally operated, however, the removal of moisture from the feed stream requires the co-permeation of significant amounts of valuable product gas. Operation of membrane systems at stage cuts on the order of 10 to 20% might be required to achieve the dewpoint level achieved by the use of a mechanical chiller. Such circumstance would, as a result, reduce the overall process recovery level achievable, increase the power requirements of the process, and be generally unattractive from an economic viewpoint. Despite such factors serving to deter the use of membrane dryer systems in place of mechanical chillers or said reversing heat exchanger and gel trap combinations, the use of membrane dryer systems in new, improved overall processes and systems, eliminating the need for the presently employed techniques, would represent a desirable advance in the art.
It is an object of the invention, therefore, to provide an improved process and system for the production of dry nitrogen and/or oxygen product.
It is another object of the invention to provide an improved process and system utilizing cryogenic systems for gas separation and providing for desired for the use of a membrane system for the removal of moisture from the feed gas.
It is a further object of the invention to provide a membrane dryer system capable of achieving enhanced drying efficiency in an overall process and system for the recovery of dry nitrogen and/or oxygen using a cryogenic system for air separation.
With those and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.